Defocus Detection Device, Defocus Detection Method and Optical Disk Unit Using the Same

ABSTRACT

Defocus detection device and method capable of detecting a defocus accurately during recording of information to an optical disk having a plurality of recording layers and an optical disk unit using the device and method are provided. A temporal restriction is imposed on defocus detection and besides, in comparison with a level for detection of a first change of a focus error signal developing during a defocus, a level for detection of a successively occurring second change of a polarity inverse to that of the first change is made to be smaller. When the first level is exceeded and thereafter the second level is exceeded within a restricted time, a defocus is detected.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/183,085, filed on Jul. 15, 2005, which claims priority from JapaneseApplication No. 2004-313213 filed on Oct. 28, 2004, the entiredisclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to detection of a defocus during recordingin an optical disk unit for recording/reproducing information to/from arecording type optical disk having a plurality of recording layers.

2. Description of the Related Art

In recent years, the recording type optical disk having a plurality ofrecording layers has been put into practice. In the event that a defocusattributable to any external disturbance takes place in the course ofrecording data onto the recording type optical disk, there is apossibility that an in-focus point of a laser beam used for recordingwill pass through another recording layer. If the in-focus point of thelaser beam passes through the different layer while the laser beam beingconditioned to emanate at recording power, an erroneous record is causedin the layer the beam transmits through. Accordingly, when a defocus isdetected during recording, the optical disk unit quickly reducesemission power of laser to a sufficiently low level to prevent anerroneous record on the different recording layer.

For detection of a defocus, JP-A-2001-67686 (Patent Document 1), forinstance, discloses a method according to which a signal indicative ofthe sum of quantities of light reflecting from an optical disk(hereinafter referred to as a SUM signal) is compared with apredetermined level.

Also, Japanese Patent No. 3456040 (Patent Document 2), for instance,discloses another defocus detection method in which a focus error signal(hereinafter referred to as a FE signal) is compared with apredetermined threshold value

SUMMARY OF THE INVENTION

Conventional technologies as above face three problems as below.

Firstly, a first issue will be described.

Referring to FIGS. 2A-2C, a lens locus, a FE signal and a SUM signalwhen an objective lens is swept in the focal direction arediagrammatically illustrated. A description will be given in connectionwith FIG. 2 by supposing that an optical disk is in rotation.

More specifically, dual recording layers L0 and L1 of the optical diskand the operation locus of the objective lens are illustrated in FIG.2A, with the recording layer L0 arranged on the side of the objectivelens. As the objective lens shown in the figure sweeps in the focaldirection while the optical disk being rotated, an in-focus point of alaser beam focused by the objective lens traces a locus as indicated bya straight line passing through point A through point D.

In such an instance, the laser in-focus point is brought into just focuson the recording layer L0 at point B and further on the recording layerL1 at point C.

As the objective lens operates in this manner, the FE signal develops asshown in FIG. 2B and similarly, the SUM signal develops as shown in FIG.2C.

Upon the instant the objective lens passes through the recording layerL0 and the instant it subsequently passes through the recording layerL1, the FE signal assumes an S-shaped waveform and the SUM signal peaksnear just focus points. The SUM signal slightly decreases in levelbetween the two recording layers.

The defocusing in the multi-layer disk is sorted into 4 kinds ofout-of-focus. More particularly, the lens defocuses from an in-focusstate on the recording layer L0 toward the objective lens as shown at anarrow (a) in FIG. 3, defocuses from the in-focus state on the recordinglayer L0 toward the recording layer L1 as shown at an arrow (b) in FIG.3, defocuses from an in-focus state on the recording layer L1 toward therecording layer L0 as shown at an arrow (c) in FIG. 3 and defocuses fromthe in-focus state on the recording layer L1 toward the opposite of theobjective lens as shown at an arrow (d) in FIG. 3.

Waveforms of FE signals and SUM signals corresponding to the 4 patternsas illustrated in FIGS. 4A-4D.

Depicted in FIG. 4A are FE and SUM signals obtained when the lensdefocuses from the in-focus state on the recording layer L0 toward theobjective lens. Similarly, FE and SUM signals obtained when the lensdefocuses from the in-focus state on the recording layer L0 toward therecording layer L1 are depicted in FIG. 4B, FE and SUM signals obtainedwhen the lens defocuses from the in-focus state on the recording layerL1 toward the recording layer L0 are depicted in FIG. 4C and FE and SUMsignals obtained when the lens defocuses from the in-focus state on therecording layer L1 toward the opposite of the objective lens aredepicted in FIG. 4D.

Since, in FIG. 4A, the laser in-focus point traces a locus from point Bto point A shown in FIG. 2A, the FE and SUM signals resemble thoseheading for point A from point B in FIG. 2A. Likewise, the FE and SUMsignals in FIG. 4B resemble those heading for point C from point B inFIG. 2A, the FE and SUM signals in FIG. 4C resemble those heading forpoint B from point C in FIG. 2A and the FE and SUM signals in FIG. 4Dresemble those heading for point D from point C in FIG. 2A.

The FE and SUM signals shown in FIGS. 4A and 4D assume waveforms similarto those developing in an optical disk having only one layer ofrecording plane and therefore in this case a defocus can be detected inaccordance with teachings of the conventional technology disclosed inthe aforementioned Patent Document 1, that is, by detecting a decreasein level of the SUM signal. On the other hand, the method of PatentDocument 1 is difficult to use in the case of in FIGS. 4B and 4C.Reasons for this will be described hereunder.

The SUM signal shown in FIG. 4B or FIG. 4C is for the inter-recordinglayer and its decreased level is smaller than that of the SUM signalshown in FIG. 4A or FIG. 4D and it is supposed that the lowermost levelof the SUM signal for the inter-recording layer is more than or higherthan half the amplitude of SUM signal. In the aforementioned PatentDocument 1, half the amplitude of SUM signal is defined as a thresholdvalue th and when the level of the SUM signal falls below the thresholdvalue th, a defocus is detected but in the case of in FIG. 4B or FIG.4C, the decreased level of the SUM signal for inter-recording layer islarger or higher than the threshold value and cannot therefore bedetected.

As a consequence, a different threshold value suitable for detection ofa small decrease in level of the SUM signal for inter-recording layerneeds to be settled and learning of the different threshold value isrequired, thus giving rise to a problem that the system is complicated.Even if the learning aids a new threshold value in its establishment,the decreased level of the SUM signal is so small thatdisadvantageously, a noise in SUM signal due to, for example, a scratchon disk surface will be detected erroneously so as to be mistaken for adefocus.

Next, a second issue will be described.

There is sometimes physical strain, caused in the course of productionprocess, in the recording surface of the optical disk. The strain has afrequency component higher than a surface deflection component of disktargeted for external disturbance suppression by servo control but thefocus servo gain is not high at the frequency of a distortional portion.In consequence, an external disturbance attributable to strain cannot besuppressed sufficiently and a change in level of the FE signal developsin the distortional portion. If in such an event the distortionalportion is large in amplitude and is high in frequency component, the FEsignal will sometimes vary to a level approximating the peak of theS-shaped waveform. In this case, the FE signal exhibits a waveform asshown in FIG. 5.

The FE signal varies to assume a waveform (a) as shown at in FIG. 5 whenthe lens sweep shown in FIG. 2A proceeds. The FE signal changes in thedistortional portion of the optical disk in various patterns supposedlyincluding a level variation of FE signal as shown at a waveform (b) or(c) in FIG. 5. In addition, the distortional portion of optical disk isnot always limited to one in number over one round of the disk andtherefore FE signal variations at waveforms (b) and (c) in FIG. 5 willsometimes be in succession.

In such an event, the method disclosed in Patent Document 2, that is,the method for detecting variations of the FE signal in both thepositive and negative levels gives rise to a problem that variations ofFE signal attributable to the distortional portion of optical disk asshown at waveforms (b) and (c) in FIG. 5 are erroneously detected so asto be mistaken for defocuses.

Next, a third issue will be described.

FIG. 6A shows a timererpart of the FE signal in FIG. 4A and FIG. 6Bshows a timererpart of the FE signal in FIG. 4C, each of the FE signalsrepresenting a FE signal which develops when the lens defocuses from arecording layer to an adjoining recording layer.

The FE signals shown in FIGS. 6A and 6B demonstrate that depending on arecording layer on which the lens is in focus during recording, the FEsignal first varies convexly in either positive polarity or negativepolarity when a defocus develops. Subsequently, the FE signal crossesthe 0 (zero) level to change convexly in the polarity inverse to that ofthe preceding change. A first reference voltage of positive leveldescribed in Patent Document 2 (Japanese Patent No. 3,456,040) as beingused for detection of a focus error signal when a defocus develops isindicated by a threshold value th1 in FIGS. 6A and 6B and a secondreference voltage of negative level similarly described as being usedfor detection of a focus error signal when a defocus develops isindicated by a threshold value th2 in these Figures.

Since in this situation the initial peak levels of the FE signalsdesignated by AMP1 in FIG. 6A and AMP2 in FIG. 6B differ from each otherin polarity but substantially equal to each other in absolute value, thethreshold values th1 and th2 may preferably be equal or substantiallyequal to each other in absolute value in order to make defocus detectionsensitivity equal for variations of the FE signal in both the positiveand negative polarities.

Further, the absolute value may preferably be half the peak level fromthe standpoint of avoiding the influence of noise in the FE signal. Whenthe techniques set forth so far are applied to an optical disk, calledblue-ray disk (hereinafter referred to as BD), using a blue laser beamfor write/read of data, problems will be entimerered as will bedescribed below. It will be appreciated that the BD of not only amono-layer BD having a single recording layer but also a double-layer BDhaving two recording layers has been put into practice.

Referring to FIG. 7, there is illustrated in sectional form adouble-layer BD. The double-layer BD has a first recording layer 20 bformed on a signal surface of a polycarbonate substrate 20 a of 1.1 mmthickness and a second recording layer 20 d with intervention of anintermediate layer 20 c. The second recording layer 20 d is covered witha cover layer 20 e of a thickness of 75 μm. In FIG. 7, an objective lensfor read/write of data is supposedly arranged beneath the disk.

If, in the optical disk unit for recording/reproducing data to/from thedouble-layer BD, an optical pickup is optically designed so that thebeam spot size of a laser beam may be optimized when the laser beam isfocused on the first recording layer 20 b, the beam spot size becomeslarger when the laser beam is brought into focus on the second recordinglayer 20 d. This is actimered for by the influence of a sphericalaberration due to the difference in distance from the disk surface tothe recording layer between the first recording layer 20 b being 100 μmdistant and the second recording layer 20 d being 75 μm distant.

Accordingly, the optical pickup incorporates a spherical aberrationcorrection device such as beam expander or liquid crystal corrector andthe spherical aberration correction device is adjustable to make thebeam spot size optimized on each recording layer. Then, when switchingthe recording layer to/from which data is written/read, the adjustmentvalue of the spherical aberration correction device must be switchedover in compliance with the targeted recording layer.

Referring to FIGS. 8A and 8B, a waveform of the FE signal developingwhen the lens is swept in the double-layer BD will be explained.Illustrated in FIG. 8A is an operation locus of the objective lens inassociation with the first and second recording layers L2 and L3 of thedouble-layer BD, indicating that the beam spot traces a straight line ADas the objective lens rises. In this procedure, the beam spot is broughtinto just focus on the second recording layer L3 at point B and on thefirst recording layer L2 at point C.

Illustrated in FIG. 8B is the FE signal when the objective lens operatesin the manner as above, exhibiting that a first S-shaped waveformdevelops around the point B and a second S-shaped waveform develops nearthe point C.

The amplitude of the S-shaped waveform developing around the point B isdesignated by AMP3 and that of the S-shaped waveform developing aroundthe point C is designated by AMP4 and signal amplitudes of the twoS-shaped waveforms are compared with each other. With the adjustment ofthe spherical aberration correction device optimized for the recordinglayer L3, the amplitude AMP4 of the S-shaped waveform developing aroundthe point C is smaller than the amplitude AMP3 of the S-shaped waveformdeveloping around the point B.

Conversely, with the spherical aberration correction device adjustedoptimally for the recording layer L2, the amplitude AMP3 of the S-shapedwaveform developing around the point B is smaller than the amplitudeAMP4 of the S-shaped waveform developing around the point C. In FIG. 8,it is assumed that the spherical aberration correction device isoptimized for the recording layer L3. Accordingly, the amplitude AMP4 issmaller in comparison with the amplitude AMP3, amounting to the half orless of the AMP3.

During recording on the recording layer L3 with the spherical aberrationcorrection device optimized for the recording layer L3, the lensdefocuses and the beam spot proceeds to the recording layer L2, so thatthe FE signal assumes a waveform resembling that over point B throughpoint D in FIG. 8A. Namely, a timererpart of the FE signal is depictedin FIG. 9, indicating that a first convex waveform having a peak levelof AMP5 first develops and successively a second convex waveform ofinverting polarity having a peak level of AMP6 develops.

In case a FE signal in excess of the peak level AMP6 again zero-crosses,causing the beam spot to pass through another recording layer, thereresults an erroneous record. It is to be noted that the AMP5 is half theAMP3 in FIG. 8B and the AMP6 is half the AMP4 in FIG. 8B.

When the technology described in Patent Document 2 is applied to thewaveform as above, the following problem will arise.

As described previously, it is desired that the threshold value th1 fordetection of the positive level of FE signal during defocusing besubstantially equal in absolute value to the threshold value th2 fordetection of the negative level of FE signal and it is further desiredthat each of the threshold values be half the amplitude of firstS-shaped signal, that is, the AMP5 in FIG. 9. Accordingly, in FIG. 9,the half of the peak level AMP5 in FIG. 9 is shown as being a thresholdvalue th3 for detection of the positive level of FE signal and theinverting-polarity level of the th3 is shown as being a threshold valueth4 for detection of the negative level of FE signal.

In this situation, the peak level AMP6 has its absolute value being lessthan half the peak level AMP5, failing to exceed the threshold value th4and a focus error signal cannot be detected. If countermeasures aretaken by making the absolute value of each of the threshold values th3and th4 half the peak level AMP6, the sensitivity to detection of FEsignal becomes high to raise a problem that the FE signal detection isrendered susceptible to the influence of noise in the FE signal.

In view of the three issues as above, the present invention has for itsobject to provide defocus detection device and method which can detect adefocus accurately and quickly by avoiding the influence of strain in anoptical disk and of spherical aberration when the defocus developsduring recording on the optical disk having a plurality of recordinglayers and an optical disk unit using the device and method.

To accomplish the above object, the present invention is constructed asbelow.

According to one aspect of this invention, a defocus detection devicefor use with an optical disk having a plurality of recording layers,comprises:

-   -   value absolutizing means for determining an absolute value of a        focus error signal;    -   first comparison means for comparing an output of the value        absolutizing means with a first predetermined value;    -   second comparison means for comparing the output of the value        absolutizing means with a second predetermined value;    -   first memory means for storing a polarity of the focus error        signal when the first comparison means delivers a signal        indicating that the output of the value absolutizing means is        larger than the first predetermined value;    -   second memory means for storing a polarity of the focus error        signal when the second comparison means delivers a signal        indicating that the output of the value absolutizing means is        larger than the second predetermined value;    -   time measurement means for measuring a predetermined time        starting to elapse after the output of the value absolutizing        means has gone beyond the first predetermined value; and    -   polarity comparison means for mutually comparing the polarities        of the focus error signal stored in the first memory means and        the second memory means,    -   wherein in a period during which the polarity comparison means        delivers a signal indicating that the two focus error signal        polarities are different and measurement by the time measurement        means continues, the output of the second comparison means is        delivered as a defocus detection signal.

Preferably, in the defocus detection device, the second predeterminedvalue may be smaller than the first predetermined value.

According to another aspect of this invention, a defocus detectionmethod for use in an optical disk unit adapted to record/reproduceinformation to/from an optical disk having a plurality of recordinglayers, comprises the steps of:

-   -   starting a defocus detection operation;    -   comparing an absolute value of a focus error signal with a first        predetermined value;    -   storing a polarity of the focus error signal when the absolute        value of the focus error signal is larger than the first        predetermined value;    -   initializing a timer value;    -   comparing the timer value with a second predetermined value to        perform the defocus detection from the beginning when the timer        value is larger than the second predetermined value and        comparing the absolute value with a third predetermined value        when the timer value is smaller than the second predetermined        value;    -   increasing the timer value and thereafter again comparing the        timer value with the second predetermined value when the        absolute value of the focus error signal is smaller than the        third predetermined value and, when the absolute value of the        focus error signal is larger than the third predetermined value,        comparing a polarity of the focus error signal at that time with        the stored polarity of the focus error signal; and    -   performing the defocus detection operation from the beginning        when the polarities are equal to each other and determining a        defocus when the polarities are different from each other.

Preferably, the third predetermined value may be smaller than the firstpredetermined value.

According to still another aspect of the invention, an optical disk unitfor recording/reproducing information to/from an optical disk having aplurality of recording layers, comprises the defocus detection device,

wherein when the defocus detection device detects a defocus, recordingto the optical disk is stopped.

According to still another aspect of the invention, an optical disk unitfor recording/reproducing information to/from an optical disk having aplurality of recording layers, comprises the defocus detection method,

wherein when the defocus detection method detects a defocus, recordingto the optical disk is stopped.

According to the defocus detection of the present invention, a highlyreliable optical disk unit capable of preventing erroneous record to adifferent layer can be provided.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a defocus detection device according to afirst embodiment of this invention.

FIGS. 2A-2C are diagrams useful to explain waveforms developing duringfocus sweep in a double-layer recording disk.

FIG. 3 is a schematic diagram showing patterns of defocus.

FIGS. 4A-4D are diagrams showing waveforms of focus error signal and sumsignal in all patterns during defocus.

FIG. 5 is a diagram showing a focus error signal waveform during focussweep and a focus error signal waveform in a distortional portion of adisk.

FIG. 6A is a waveform diagram of a focus error signal developing whenthe lens defocuses toward a different recording layer.

FIG. 6B is a similar waveform diagram of an inverted focus error signal.

FIG. 7 is a schematic sectional diagram of a double-layer blue-ray disk.

FIGS. 8A and 8B are diagrams useful to explain a focus error signalwaveform during focus sweep in the presence of a spherical aberration.

FIG. 9 is a waveform diagram of a focus error signal when the lensdefocuses in the presence of a spherical aberration.

FIGS. 10A-10G are waveform diagrams useful to explain operation of thefirst embodiment of this invention.

FIGS. 11A-11G are waveform diagrams useful to explain another type ofoperation of the first embodiment of this invention.

FIG. 12 is a flowchart showing a second embodiment of this invention.

FIG. 13 is a diagram of a focus error signal waveform developing whenthe lens defocuses in an optical disk of three or more layers.

FIG. 14 is a block diagram showing a third embodiment of this invention.

FIG. 15 is a block diagram showing a fourth embodiment of thisinvention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Embodiments in terms of hardware and of software will be describedhereunder.

Embodiment 1

A first embodiment of this invention in terms of hardware will first bedescribed with reference to a block diagram of FIG. 1.

A FE signal 1 is a signal generated by using an output of an opticalpickup, not shown, in an optical disk unit through known astigmatismmethod or knife edge method and it indicates a positional error,referenced to the recording surface of an optical disk, of a laser spotin the optical axis direction.

A value absolutizing circuit 2 delivers an absolute value of FE signal 1referenced to a reference level. The output of the value absolutizingcircuit is connected to non-inverting input terminals of comparisoncircuits 5 and 6.

A first threshold level voltage 3 is set to a level th1 for detectingthat the output of value absolutizing circuit 2 exceeds a firstpredetermined voltage. The setting level th1 is set with a predeterminedvalue which permits detection of a first peak of change in level of theFE signal when a defocus develops.

A second threshold level voltage 4 is set to a level th2 for detectingthat the output of value absolutizing circuit 2 exceeds a secondpredetermined voltage. A predetermined value set to the setting levelth2 permits detection of a second peak of change in level of the FEsignal when the defocus develops.

It will be appreciated that the second peak of FE signal during thedefocusing is smaller than the first peak level under the influence of aspherical aberration. Therefore, the setting value th2 of secondthreshold value voltage is less than the setting value th1 of firstthreshold value voltage. Supposedly, in the present embodiment, thesetting value th2 of second threshold value voltage is half the settingvalue th1 of first threshold value voltage.

The comparison circuit 5 includes its non-inverting input terminalconnected to the output of value absolutizing circuit 2 and itsinverting input terminal connected to the output of first thresholdvalue voltage 3. The comparison circuit 5 delivers a high (Hi) levelwhen the output signal level of value absolutizing circuit 2 exceeds thefirst threshold value voltage and a low (Low) level when that outputsignal level is less than the second threshold value voltage.

A memory circuit 7 stores a polarity, referenced to a reference voltage,of the FE signal at the timing that the output of comparison circuit 5turns to the Hi level. By rule, the Hi level is stored when the positivepolarity prevails with the Low level stored for the negative polarity.

A memory circuit 8 stores a polarity, referenced to the referencevoltage, of the FE signal at the timing that the output of comparisoncircuit 6 turns to the Hi level. By rule, the Hi level is stored whenthe positive polarity prevails with the Low level stored for thenegative polarity.

A polarity decision circuit 9 compares the polarities of FE signalstored in the memory circuits 7 and 8 and delivers a Hi level when thepolarities differ from each other and a Low level for the polaritiesbeing equal.

A first logical product circuit 10 delivers a logical product of outputsof the polarity decision circuit 9 and comparison circuit 6.

A monostable multivibrator circuit 11 delivers a Hi level signal onlyfor a predetermined period starting to elapse after the output of thecomparison circuit 5 has turned to the Hi level.

A second logical product circuit 12 delivers a logical product ofoutputs of the first logical product circuit and monostablemultivibrator circuit 11.

A defocus detection signal 13 is delivered out of the second logicalproduct circuit 12.

With the construction as above, the defocus detection device operates aswill be described below with reference to FIG. 10 showing outputwaveforms of individual blocks in FIG. 1.

Illustrated in FIG. 10A is a FE signal when a defocus develops whichindicates that with the lens defocused, the FE signal first assumes apositive-going change exceeding the reference level to exhibit a firstconvex change. Thereafter, the FE signal assumes a negative-going changewhich zero-crosses to fall below the reference level, exhibiting asecond convex change. Supposedly, in FIG. 10A, comparison of the firstand second convex changes shows that the peak level of first convexchange is twice in absolute value that of second convex change.

The output of the value absolutizing circuit 2 is depicted in FIG. 10B.This output goes beyond the setting level th1 of first threshold valuevoltage 3 shown at dotted line in the figure during a period rangingfrom time t1 to time t2. Further, during a period of from time t3 totime t4, the output is in excess of the setting level th2 of secondthreshold value voltage 4 shown at dotted line in the figure.

The output of the comparison circuit 5 is depicted in FIG. 1C, assumingthe Hi level for the period (from time t1 to time t2) during which theoutput of value absolutizing circuit 2 shown in FIG. 10B goes beyond thesetting level th1 of first threshold value voltage 3. In this phase, theFE signal is in excursion of positive polarity and hence the memorycircuit 7 stores the Hi level.

The output of the comparison circuit 6 is depicted in FIG. 10D, assumingthe Hi level for the period (from time t3 to time t4) during which theoutput of value absolutizing circuit 2 shown in FIG. 10B goes beyond thesetting level th2 of second threshold value voltage 4. At that time,since the FE signal is in excursion of negative polarity, the memorycircuit 8 stores the Low level.

Illustrated in FIG. 10E is the output of first logical product circuit10. The polarities of the FE signal stored in the memory circuits 7 and8 are different corresponding to the Hi level and Low level,respectively, and therefore, the output of polarity decision circuit 9representing one input to the first logical product circuit 10 assumesHi level. As a consequence, the first logical product circuit 10delivers the same output as that at (d) of the second comparison circuit6.

Illustrated in FIG. 10F is the output of monostable multivibratorcircuit which is at Hi level for a predetermined period of Tg followingthe time t1 that the first comparison circuit 5 turns to the Hi level.The period Tg for delivery of the Hi level is preferably about 5 ms.

The output of second logical product circuit 12, that is, the defocusdetection signal 13 is depicted in FIG. 10G. The defocus detectionsignal 13 assumes the Hi level for the period during which the output ofmonostable multivibrator shown in FIG. 10F is at the Hi level and at thesame time the output of second logical product circuit 10 shown in FIG.10E is at the Hi level. In other words, the defocus can be detected attime t3.

In the defocus detection device operating in the way as above, theabsolute value of the FE signal is monitored to detect the first convexchange over the period of from t1 to t2 and thereafter when the secondconvex change of opposite polarity whose duration is from t3 to t4 isdetected within the predetermined period Tg, this change is detected asa signal indicative of the defocus.

The waveforms shown in FIGS. 10A-10G are useful to explain a defocustracing from point C to point B in FIG. 8A but waveform developing whena defocus traces from point C to point B are illustrated in FIG. 11. Itis supposed in FIG. 11 that the spherical aberration correction deviceis adjusted optimally for the recording layer L2.

Illustrated in FIG. 11A is a FE signal developing during the defocustracing from point C to point B in FIG. 8A, having its polarity oppositeto that of the FE signal shown in FIG. 10B. Further, the output of valueabsolutizing circuit 2 is depicted in FIG. 11B as being the same as thatin FIG. 10B because the signals in FIG. 10A and in FIG. 11A are the samewith only exception of their polarities. Likewise, waveforms in FIGS.11C-11G are the same as those in FIGS. 10C-10G.

As will be seen from the above, even when the direction of a defocusdiffers, the polarity of FE signal in FIG. 11A alone is inverted withthe remaining signal waveforms intact and as in the case of FIG. 10A,the defocus can be detected at time t3.

Since in the defocus detection device according to the first embodimentset forth so far, a temporal restriction by dint of the monostablemultivibrator circuit is imposed on the defocus detection, the defocusdetection is hardly affected by noises in FE signal attributable tostrain in the disk. Further, by making the threshold value voltage fordetection of the second convex change of the FE signal developing afterthe defocus smaller than the threshold value voltage for detection ofthe second convex change, the influence of the spherical aberration canbe avoided.

Furthermore, structurally, the defocus detection signal may be inputtedto a system controller of the optical disk unit and the systemcontroller may instantaneously stop recording of data when a defocus isdetected on the basis of the defocus signal, thereby making it possibleto prevent an erroneous recording on a different layer. Thanks to this,a highly reliable optical disk unit can be provided.

Embodiment 2

Next, a second embodiment of this invention will be described. While inthe foregoing first embodiment the defocus detection method has beendescribed in terms of hardware, the defocus detection will be describedin terms of software in the second embodiment.

Operation will now be described with reference to a flowchart of FIG.12.

When a defocus detection operation is started (S1), the systemcontroller in the optical disk unit monitors a FE signal to decidewhether the absolute value of the FE signal exceeds the first thresholdvalue th1 (S2) and if the absolute value is determined as being lessthan the threshold value th1 (No), the system controller continuesmonitoring a FE signal. If the absolute value is determined as goingbeyond the threshold value th1 (Yes), the system controller stores apolarity of the FE signal at that time (S3) and initializes the timererbuilt in the system controller to zero (0) (S4). In other words, theabove steps S3 and S4 are executed at timing corresponding to the timet1 at which the absolute value of the FE signal shown at (b) in FIG. 10exceeds the threshold value th1.

In addition, it is decided whether a timer value delivered out of thetimerer goes beyond the predetermined value Tg (S5). If the timer valueexceeds Tg (Yes), a FE signal is again monitored (S2). If the timervalue is less than Tg (No), the FE signal is monitored to decide whetherits absolute value is in excess of the second threshold value th2 (S6)and with the absolute value determined as being less than the thresholdvalue th2 (No), the timer value is upped (S7) and a resulting timervalue is again compared with the predetermined value Ta (S5). To add,since a series of operations starting from the step S5 and ending in thestep S7 is completed within the predetermined period, the timer value inexcess of the predetermined value Tg signifies that the predeterminedperiod is exceeded and the measuring time is over.

If the absolute value of FE signal exceeds the second threshold valueth2 (Yes), the polarity of FE signal precedently stored is compared withthat of FE signal at present (S8). In other words, the timing for thestep S8 to be executed corresponds to time t3 from which the absolutevalue of FE signal goes beyond the threshold value th2. When thecomparison result shows that the two polarities are equal (Yes), adefocus is not determined and monitoring of a FE signal again proceeds(S2).

Contrarily, with the two polarities determined as being different fromeach other (No), a defocus is determined and a defocus detection processis executed (S9) and then the series of operations ends (S10). Thedefocus detection process (S9) is necessary in the event of occurrenceof a defocus to perform such an operation as stopping data recording oronce turning off the servo control and then restarting it.

Through the above operation, the defocus can be detected at time t3 inFIG. 10 and therefore the system controller can undertake the necessaryprocess such as stopping of data recording.

While the foregoing embodiments of this invention have been described byway of the double-layer disk, the present invention can also be appliedsimilarly to an optical disk of three or more layers. Conceivably, theFE signal in the case of a defocus developing in the double-layer diskas shown in FIG. 9 can be applicable to an optical disk of M layers(M>3) as shown in FIG. 13.

More specifically, as the lens defocuses to sequentially go acrossdifferent layers in succession, the spherical aberration graduallyincreases and responsive thereto, the change level of a focus errorsignal decreases gradually. Since in FIG. 13 the beam spot is broughtinto just focus on an adjacent layer at a black dotted position, adefocus must be detected at a timing earlier than this black dottedposition in order to prevent an erroneous recording.

Embodiment 3

An example of a product to which the defocus detection device describedin embodiment 1 or the defocus detection method described in embodiment2 is applied will be described.

Turning now to FIG. 14, a personal computer is illustrated in blockdiagram form. The personal computer designated at reference numeral 21in FIG. 14 comprises constituent components given below.

A central processing unit (hereinafter simply referred to as CPU) 22processes various kinds of information so as to input/output datato/from individual peripheral equipments coupled to the CPU.

A main storage 23 stores data necessary for the CPU to processoperations. Generally, a synchronous dynamic random access memory(SDRAM) is used as the main storage.

A first interface circuit 24 intervenes to connect the CPU 22 to themain storage 23.

A hard disk drive 25 records various kinds of information.

A second interface circuit 26 intervenes to connect the CPU 22 to thehard disk drive 25.

An optical disk unit 27 includes an optical pickup, a loading mechanismunit, a spindle motor and a control circuit for controlling them and itoperates to reproduce information from an optical disk and recordinformation onto the optical disk.

Denoted by 28 is a defocus detection circuit corresponding to thedefocus detection device described in connection with embodiment 1 or amicrocomputer for execution of the defocus detection method described inconnection with embodiment 2. The defocus detection circuit 28 issupplied with a focus error signal 29 from the optical disk unit 27.Conversely, the defocus detection circuit 28 supplies a defocusdetection signal 30 to the optical disk unit 27. The defocus detectionsignal 30 connects to the control circuit inside optical disk unit 27 toserve as an interrupt signal.

The personal computer 21 may additionally comprise a user input/outputinterface equipment such as keyboard, mouse or display not shown.

With the construction as above, when, for example, information stored inthe hard disk drive 25 is to be recorded on the optical disk by means ofthe optical disk unit, the CPU 22 reads the target information from thehard disk drive 25 through the second interface circuit 26 and stores itin the main storage 23 through the first interface circuit 24. Then, theCPU 22 sends the information stored in the main storage 23 to theoptical disk unit 27 via the first and second interface circuits andcommands the optical disk unit 27 to perform a recording operation.

In the event that any external disturbance such as, for example, avibration is applied to the optical disk unit 27 during recording ofinformation onto the optical disk and a defocus toward another layerdevelops, the defocus detection circuit 28 detects the defocus. As aconsequence, the defocus detection circuit 28 outputs, as the defocusdetection signal 30, a signal resembling that shown in FIG. 10G or FIG.11G to apply interrupt processing to the control circuit inside opticaldisk unit 27.

Consequently, the control circuit inside optical disk unit 27 detectsthe defocus and immediately stops the operation of recording onto theoptical disk and besides turns off both of the focus servo and trackingservo. In this manner, even when the defocus develops, the optical diskunit 27 can be kept from committing an erroneous record onto anotherrecording layer.

Thereafter, the control circuit inside optical disk unit 27 again turnson the focus servo and tracking servo to position the optical pickup ata location immediately preceding the recording stoppage and resumes therecording operation temporarily deactivated till then. In this manner,the information can be recorded on the optical disk without midwayinterruption. In addition, the defocus can be dealt with by means ofonly the optical disk unit 27 and defocus detection device 28, with theresult that the load on the CPU 22 can be kept from increasing and theCPU 22 can execute other applications without interruption.

The present embodiment has been described by way of example of thepersonal computer but this invention can also be applied in a similarway even to another computer system such as work-station or mainframe,provided that it includes the optical disk unit 27 and defocus detectioncircuit 28 as constituent components.

While in FIG. 14 the optical disk unit 27 and defocus detection circuit28 are illustrated as being separate components, the defocus detectioncircuit 28 may be incorporated in the optical disk unit 27.

Further, in place of the hard disk drive 25, another type of memorydevice for use with an optical disk unit other than the optical diskunit 27 or a magneto optical disc (MO) unit may be used.

Embodiment 4

Another example of a product to which the defocus detection devicedescribed in embodiment 1 or the defocus detection method described inembodiment 2 will be described.

A video recording apparatus is illustrated in block diagram form in FIG.15. Denoted by 31 is the video recording apparatus which comprisesconstituent components given below. The same constituents as those inFIG. 14 are designated by the same reference numerals and will not bedescribed herein.

An analog broadcast input terminal 32 receives a reception signal ofground wave analog broadcast.

An analog broadcast tuner 33 selects and outputs a video signal of agiven broadcasting station.

An external input terminal 34 receives a video signal delivered out ofanother video equipment.

A first selection circuit 35 selects and outputs either the video signaldelivered out of the analog broadcast tuner 33 or the video signalinputted to the external input terminal 34.

An encoder circuit 36 encodes the video signal the first selectioncircuit 35 outputs into a predetermined format to deliver streamingdata. Supposedly, the video signal in the present embodiment is in MPEG2format.

A ground wave digital broadcast input terminal 37 receives a receptionsignal of ground wave digital broadcast.

A ground wave digital broadcast tuner 38 selects a video signal of adesired broadcasting station from the reception signal inputted to theground wave digital broadcast input terminal and outputs the selectedsignal in the form of streaming data. Supposedly, videos of ground wavedigital broadcast are in MPEG2 format.

A second selection circuit 39 selects and outputs either the streamingdata delivered out of the encoder circuit 36 or the streaming datadelivered out of the ground wave digital broadcast tuner 38.

A buffer circuit 40 is used for buffering of the streaming data ofsecond selection circuit 39 and of streaming data an optical disk unit27 or a hard disk drive (HDD) 41 to be described later delivers.

Saved in the HDD 41 are video data.

A CPU 42 controls the second selection circuit 39, buffer circuit 40,optical disk unit 27 and HDD 41 in order to control input/output ofstreaming data to/from the buffer circuit 40.

It will be appreciated that a focus error signal 29 is supplied from theoptical disk unit 27 to a defocus detection circuit 28 and a defocusdetection signal 30 is supplied from defocus detection circuit 28 tooptical disk unit 27. The defocus detection signal 30 connects to thecontrol circuit inside optical disk unit 27 to serve as an interruptsignal.

With the construction as above, recording of videos of analog broadcast,for example, onto the optical disk is carried out as will be describedbelow.

A video signal of analog broadcast inputted to the analog broadcastinput terminal 32 is passed through the first selection circuit 35 andencoder circuit 36 so as to be converted into streaming data which inturn is passed through the second selection circuit 39 so as to undergobuffering by means of the buffer circuit 40. With the amount ofstreaming data buffered in the buffer circuit 40 to some extent, the CPU42 commands the optical disk unit 27 to record streaming data insidebuffer circuit 40 onto an optical disk. In this manner, a video ofanalog broadcast can be recorded on the optical disk.

It will be appreciated that a video signal received at the externalinput terminal 34 and passed through a different path preceding thefirst selection circuit 35 and a video signal received at the groundwave digital broadcast input terminal 37 are processed, as in theprecedence, in the second selection circuit 39 and succeeding stage soas to be recorded on the optical disk.

In dubbing a video recorded in the HDD 41 onto the optical disk,streaming data in the HDD 41 is buffered in the buffer circuit 40. Then,when the amount of data in the buffer circuit 40 comes up to someextent, the CPU 42 commands the optical disk unit 27 to record streamingdata in buffer circuit 40 onto the optical disk. In this manner, thevideo in the HDD 41 can be dubbed onto the optical disk.

As will be seen from the above, various kinds of input videos and videosrecorded on the HDD 41 can be recorded on the optical disk.

In the event that any external disturbance such as a vibration isapplied to the optical disk unit 27 during recording of video data ontothe optical disk and a defocus toward another layer develops, thedefocus detection circuit 28 detects the defocus. As a consequence, thedefocus detection circuit 28 outputs, as the defocus detection signal30, a signal resembling that shown in FIG. 10G or 11G to apply interruptprocessing to the control circuit inside optical disk unit 27.

Consequently, the control circuit inside optical disk unit 27 detectsthe defocus and immediately stops the operation of recording onto theoptical disk and besides turns off both of the focus servo and trackingservo. In this manner, even when the defocus develops, the optical diskunit 27 can be kept from committing an erroneous record onto anotherrecording layer.

Thereafter, the control circuit inside optical disk unit 27 again turnson the focus servo and tracking servo to position the optical pickup ata location immediately preceding the recording stoppage and resumes therecording operation temporarily deactivated till then. This ensures thatthe information can be recorded on the optical disk without midwayinterruption. Further, the defocus can be dealt with by means of onlythe optical disk unit 27 and defocus detection circuit 28 and the loadon the CPU 22 can be kept from increasing, with the result that the CPU22 can execute another type of operation, for example, follow-upreproduction without interruption.

In FIG. 15, the optical disk unit 27 and defocus detection circuit 28are illustrated as being separate constituent components butalternatively, the defocus detection circuit 28 may be incorporated inthe optical disk unit 27.

According to the present invention, the defocus can be detected at thetiming earlier than the black dotted position and consequently, a focuserror signal waveform appearing after the black dot is meaningless. Inthis manner, according to the present invention, the defocus can bedetected even in the multi-layer optical disk of three or more layersthrough the same operation as that for the double-layer optical disk.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims

1. A defocus detection device for use with an optical disk having aplurality of recording layers, comprising: first detection means fordetecting that a focus error signal goes beyond a first predeterminedlevel; second detection means for detecting that said focus error signalexceeds a second predetermined level of a polarity opposite to that ofthe first predetermined level; and time measurement means for measuringa time starting to elapse after the focus error signal has gone beyondsaid first predetermined level, wherein when said second detection meansdetects that the focus error signal exceeds said second predeterminedlevel before said time measurement means measures a predetermined time,a defocus detection signal is outputted.
 2. A defocus detection deviceaccording to claim 1, wherein said second predetermined level has anabsolute value smaller than that of said first predetermined level. 3.An optical disk unit for recording/reproducing information to/from anoptical disk having a plurality of recording layers, comprising thedefocus detection device as recited in claim 1, wherein when saiddefocus detection device detects a defocus, recording to said opticaldisk is stopped.